The present invention relates to an echo canceling apparatus useful in a hands-free communication device such as a teleconferencing terminal or a mobile telephone set mounted in an automobile.
Referring to FIG. 1, in a conventional echo canceling apparatus of the type used in a hands-free telephone set, an incoming far-end speech signal Rin, received as an electrical signal at an input terminal 11, is reproduced through a loudspeaker SP as a far-end acoustic signal AS2. A near-end acoustic signal AS1, carrying the voice of a near-end party or user U, is picked up by a microphone M and converted to an electrical speech signal S. The microphone M also picks up an echo of the far-end acoustic signal AS2, which is reflected by the near-end party U and other objects in the ambient environment. The microphone output signal S thus includes an undesired far-end echo component as well as the desired near-end voice component.
The microphone output signal S is supplied to an echo canceler EC having an adaptive filter ADF that generates an echo replica Res from the received far-end signal Rin. A subtractor 13 (shown as an adder with a minus sign beside the subtrahend input) subtracts the echo replica Res from the microphone output signal S to obtain a residual signal E. Ideally, the echo replica Res exactly matches the echo of the far-end signal, so that the echo is completely removed from the residual signal E. The residual signal E is supplied to an output terminal 12, to be transmitted to the far-end party as an outgoing speech signal Sout.
The adaptive filter ADF has tap coefficients that are updated with reference to the residual signal E, to adapt to changes in propagation characteristics of the echo path. The updating takes place under the control of a talk state detector or double-talk detector DTD, which determines the current near-end and far-end talking states by comparing signals Rin and E, or Rin, E, and S. Updating of the tap coefficients is generally limited to the receive single-talk state, in which only the far-end party is talking. The updating algorithm attempts to minimize the power of the residual signal E in this state. During the double-talk state, in which both parties speak at once, the tap coefficients are held unchanged, but the adaptive filter continues to generate an echo replica Res, which is subtracted from the outgoing speech signal, so that the far-end party does not hear an echo of his or her own voice.
A problem is that the near-end acoustic environment may include a noise source NS, which generates a background acoustic noise signal AS3. The noise signal AS3, which is also picked up by the microphone M, cannot be replicated by the adaptive filter ADF, because it is unrelated to the far-end signal Rin. Accordingly, the echo canceler EC does not remove near-end background noise from the outgoing speech signal Sout. The background noise also interferes with the tap-coefficient updating algorithm, so that the echo replica Res does not match the actual far-end echo component, and some residual echo remains in the outgoing speech signal Sout. In general, the echo canceler EC cannot reduce the residual echo level below the background noise level.
Thus despite echo cancellation, the outgoing speech signal is contaminated by background noise and uncanceled echo, and does not have as high a signal-to-noise ratio as might be desired.
An object of the present invention is to improve the output signal-to-noise ratio of an echo canceling apparatus.
The invented method of canceling echo includes the steps of:
receiving an acoustic signal, including a near-end component and an echo component, at a first microphone and a second microphone, obtaining a first output signal from the first microphone and a second output signal from the second microphone;
using a first adaptive filter to cancel the echo component in the first microphone output signal, obtaining a first residual signal;
using a second adaptive filter to cancel the echo component in the second microphone output signal, obtaining a second residual signal;
determining a timing delay between the near-end components in the first and second residual signals; and
additively combining the first and second residual signals according to the timing delay to obtain an outgoing signal.
The timing delay is determined from, for example, a timing difference between detection of the near-end components in the first and second microphone output signals, a timing difference between echo delay times of artificially generated acoustic reference signals, a difference between the positions of peak tap coefficients in the first and second adaptive filters, or a sliding correlation of the first and second residual signals.
The invention also provides an echo canceling apparatus having a pair of microphones, a corresponding pair of echo cancelers with respective adaptive filters, and a combining unit combining the residual signals from the two echo cancelers according to a timing delay as described above.
The signal-to-noise ratio of the outgoing signal is improved because when the two residual signals are combined, the two near-end components reinforce each other, while other components become relatively weakened.